As a means for detecting a trace of a specimen in a sample, the immunoassay method has been heretofore known. Among other methods, the ELISA (enzyme-linked immunosorbent assay) method has been widely disseminated owing to its capability of implementing the detection with high sensitivity. The ELISA method nevertheless entails such problems as necessitating protraction of the time for an operation and the time for a reaction and as well complicating the operation of determination.
In recent years, therefore, as immunoassay methods that promise to take the place of the ELISA method, the immunochromatography method that comprises causing a test solution to be absorbed in one end of a membrane having an antibody fixed thereon and enabling the test solution to be gradually spread in the lateral direction by means of capillarity and the flow-through method that causes a membrane having an antibody fixed thereon to pass a test solution therein in the direction of thickness of the membrane have been attracting attention. In particular, the immunochromatography method excels in various points such as smallness and eminent portability of an apparatus (strip) and superiority over the ELISA method in stability of preservation, rapidity of determination, easiness of judgment, and obviation of the necessity for a special attachment. Thus, the immunochromatography method is popularly used in the field of diagnostic drugs for testing patients for influenza, for HCV, for pregnancy, for allergy, and for food poisoning, for example.
The immunochromatography method that utilizes a sandwich process is carried out as follows, for example. First, two kinds of antibodies capable of recognizing different regions of a specimen and a strip of membrane are prepared; one kind of antibody (capturing antibody) is immobilized in advance in a region of the membrane called a test line and the other kind of antibody is transformed, via labeling with colloidal particles, for example, into a gold colloid-conjugated antibody. Then, a test solution is mixed with the gold colloid-conjugated antibody and the strip is caused to absorb the resultant mixture in one end thereof and spread it therein. When the test solution happens to contain a specimen, the specimen reacts with the gold colloid-conjugated antibody to form a specimen-gold colloid-conjugated antibody complex and this complex, while passing on the test line, is captured by the capturing antibody to form a capturing antibody-specimen-gold colloid-conjugated antibody complex. As a result, the gold colloid-conjugated antibody is observed to develop a red color on the test line.
The immunochromatography method mentioned above embraces a widely known process that comprises labelling an antibody with such an enzyme as an alkali phosphatase or a peroxidase in the place of the colloidal gold particles and having a color developing agent subsequently spread in the marked antibody. Patent Document 1, for example, proposes a method for carrying out enzymatic immunoassay in the presence of a base in the determination of enzymatic immunity using a peroxidase. Then, Patent Document 2 proposes an apparatus for chromatographic assay where an antibody-immobilized part having immobilized thereon an antibody capable of specifically combining with a specimen under assay and a pigment-immobilized part having an optional pigment immobilized thereon coexist at one position    Patent Document 1: JP-A 2001-0074740    Patent Document 2; JP-A 2001-004613